Reflecting mirror and lens objective of the cassegrain type



JDU-Q C Dec. 13, 1955 w. MANDLER 2,

REFLECTING MIRROR AND LENS OBJECTIVE OF THE CASSEGRAIN TYPE Filed Aug.30, 1951 O l O QINVENTOR.

Walter Mend/er United States Patent REFLECTING MIRROR AND LENS OBJECTIVEOF THE CASSEGRAIN TYPE Walter Mandler, Atzbach, near Wetzlar, Germany,as-

signor to Ernst Leitz, G. m. b. 11., a corporation of GermanyApplication August 30, 1951, Serial No. 244,376

Claims priority, application Germany September 11, 1950 3 Claims. (Cl.88-57) This invention relates to improvements in reflecting lensobjectives which are characterized by having a great focal length, 600mm. for example; an aperture opening in the proportion of 1:5; adistance of the image from the last lens vertex of at least 113 mm., andan useful angle of vision of 4 degrees. Such objectives are particularlyuseful for photographic purposes.

It is known, that for a correction element of the Schmidt type, whichoften is used for eliminating the image defects of a spherical mirror, acorrection element consisting of spherical surfaces may be substituted.The Swiss Patent 255,151 describes such an objective in which acorrection element is placed in front of the spherical mirror, saidelement comprising a meniscus and a positive lens, both of which aremade of the same kind of glass and together yield a low positiverefractive power. If these refractive powers are low enough, it ispossible, by using only one kind of glass, to provide a mirror which isalmost without chromatic defects.

The object of the invention is to utilize such known means for theconstruction of an objective of the Cassegrain type. The invention isembodied in such an objective in which one correction element is placedin the path of the light rays in front of the collecting main mirror,and a second correction element is placed behind the second dispersingmirror. Each of the said correction elements consists of two sphericallenses positioned close together. The retracting power of each elementis zero and all the lenses are madeof the same kind of glass.

In the accompanying drawing Figures 1 and 2 are diagrams illustratingobjectives embodying the invention in accordance with the data containedin the table of values appearing hereinafter in the'description of eachof the figures in the drawing and also included in the claims, whichshould be read accordingly. In the tables the radii of curvature, thelens thicknesses, the axial distances between lens elements and otherdimensional and optical details are specified and enumerated, namelythat r=the radius d=thickness of lens r to r=distance between lensesn=refractive index f=focal length of the system The aperture proportionand the angle of vision are also set forth.

In the drawing the main mirror 1 is concave, the mirror 2 is convex. Thefirst correction element comprises a meniscus 3 and a lens 4. The secondcorrection element comprises the lenses 5 and 6. The numerals 7 and 7'indicate screening members for preventing the passage of undesirablelight rays.

2,726,574 Patented Dec. 13, 1955 Fig. 1 illustrates a reflecting lensobjective embodying the invention in which the several elements shownare characterized by the following values:

Thicknesses Distances n 659. 1 n to n- 15.7 d =13. 6 n to n: 9. 4 T:=+1669. 7 T5 to 125. 5 7'3 296. 1 n 20 f =l34.9 d1=l1.5 n to r =125.5 r413.8 rs to n: 3. 1 rs 444. 2 rs 307.8 r1 104.6

ds= 4. 2 rs 81. 6 n 221.8

di= 8.3 rm=+ 429.8

Distance of the image from the last lens verter==123 mm. Aperatureproportlon- 1:5.

Angle of vlsion=4 degrees.

Fig. 2 illustrates another reflecting lens objective embodying theinvention in which the several elements are characterized by thefollowing values.

Thicknesses Distances r =+476. 9 n to fa= 21.0

d =13. 6 n to n= 0 n =+712. 6 n to 11-1258 1'; =249. 3 n to n-l25.8ds=l2.fi h to T1=125 3 r =308.1 n to n: 3 1 r5 -444. 9 =308. 1 r1=+104.8

d 4. 2 rs 81 7 n =+a09 4 Distance of the image from the last lensvertex=121.75 mm. Aperture proportion-l :5.

Angle of vislon=4 degrees.

I claim:

1. A reflecting lens objective of the Cassegrain type comprising a frontcorrection element consisting of a front spherical concave lens and arear spherical convex lens placed close together; a collecting mainmirror behind said front correction element; a dispersing mirror behindsaid collecting main mirror spaced a relatively great distancetherefrom, said collecting main mirror being provided with an axiallydisposed aperture; a second rear correction element consisting of twospherical lenses placed close together and positioned behind saiddispersing mirror in axial alinement with the said aperture in saidcollecting main mirror; each of said two correction elements having aretracting power of zero and all of said lens elements being made of thesame kind of glass; and screening members between the said collectingmain mirror and said dispersing mirror to exclude the passage ofundesirable light rays through the said objective.

2. A reflecting lens objective of the Cassegrain type in which onecorrection element is positioned in front of the collecting mirror and asecond correction element is positioned behind the second mirror, eachof said elements consisting of two spherical lenses placed closetogether, each of said correction elements having a refracting power ofzero and all of said lenses being made of the same kind of glasscharacterized by the following data:

Thickneeses Distances r 659. 1 n to rs= 15.7 111-134 6 T4 t rs= 9. 4 r1-+1660. 7 rs to ra=125. h 296.1 T4t07r=l34.9 d =11. 5 n to T1==l25. 5 r413.6 n to n- 3. 1 n 444. 2 I 12 7 1 d;- L 2 7 81. 8 n 221.8

r-radius. d-thickness oi lens.

r to rdlstance between lenses.

Refractive index n-1.514/64.

Focal length t=600 mm.

Distance of the image from the last lens vertex=123 mm. Apertureproportion-1:5.

Angle of vision=4 degrees.

3. A reflecting lens objective of the Cassegrain type in which onecorrection element is positioned in front of the collecting mirror and asecond correction element is positioned behind the second dispersingmirror, each of said elements consisting of two spherical lenses placedclose together, each of said correction elements having a refractingpower of zero and all of said lenses being made of the same kind offollowing data:

'lhicknesses Distances 5 n =+476.9 r: to T3= 21.0

d =13.6 n to n= 0 r; =+712. 6 r5 to n=125. 8 n -249.3 m to n=125.8dz=12.6 rsto r1=l25.8 r ==308.1 Ts to n= 3.1 10 T5 =444. 9 Zifii' a,-4.2 H Z n t as 'l r=radius. d=thlckness 0! lens. r to r=distance betweenlenses. Refractive index 11:1.614/64. Focal length l-600 mm. Distance ofthe image from the last lens vertex-121.75.

Aperture proportion=l:5. Angle oi vision-4 d.

References Cited in the file of this patent UNITED STATES PATENTS2,403,660 Hayward July 9, 1946 2,420,349 Bouwers May 13, 1947 2,492,461Bouwers Dec. 27, 1949 2,504,383 Bouwers et al Apr. 18, 1950 2,520,635Grey Aug. 29, 1950 FOREIGN PATENTS 618,253 Great Britain Feb. 18, 1949969,797 France May 31, 1950 glass characterized by the

